1. Field of the Invention
The present invention relates in general to a rubber composition, and in particular to such a rubber composition comprising a diene rubber material and vulcanization accelerators.
2. Discussion of the Prior Art
Molding and vulcanization of a rubber material or composition need thermal energy, unlike forming of a plastic or resin material. The cycle time of molding and vulcanization of a rubber material can be shortened to generally the same as that of forming of a resin material, by reducing the vulcanizing time of the rubber material.
The so-called "high-temperature and short-time" vulcanization method is one of the known vulcanization methods in which the vulcanizing time is reduced. In this method, vulcanization is performed at higher temperature so as to enhance reaction between a vulcanizing agent and a rubber material. However, since the rubber material is subjected to high temperature, reversion tends to take place. If reversion takes place, rubber articles produced from the rubber material suffer from deteriorated properties, specifically modulus. In particular, in the case of a rubber material for a rubber vibration isolator, in which the rubber material is adhered by vulcanization to a metal member to which a vulcanizable adhesive has been applied, the rubber vibration isolator produced suffers from lowered adhesive force between the vulcanized rubber and the metal member. Thus, the rubber vibration isolator as an end product is not satisfactory in qualities, specifically in these two respects; one is the properties of the vulcanized rubber and the other is the adhesive force between the vulcanized rubber and the metal member.
Meanwhile, various vulcanization accelerators have conventionally been used for the purpose of reducing vulcanizing time or increasing vulcanization rate without having to raise vulcanizing temperature. The known vulcanization accelerators are such as; sulfenamide type accelerators, thiazole type accelerators, thiuram type accelerators, dithiocarbamates, and xanthates. Above all, the thiuram type accelerators are very excellent vulcanization accelerators. Vulcanizing time can be reduced to a significantly short time by using a thiuram type accelerator solely or in combination with a sulfenamide type accelerator.
However, the use of a thiuram type accelerator leads to deteriorating the properties of the vulcanized rubber, in particular extremely deteriorating tear strength and elongation thereof. If a thiuram type accelerator is used for vulcanizing a rubber material to produce a rubber vibration isolator, the rubber vibration isolator as an end product suffers from reduced durability.
Thus, it has conventionally been difficult to reduce the vulcanizing time without deteriorating the qualities of the rubber articles or products.
The requirement that the vulcanizing time be reduced is remarkably high in the field of manufacture of rubber products having a large rubber volume, specifically rubber vibration isolators for engine mounts. In the field of the engine mounts, another method has been employed for reducing the vulcanizing time, wherein a rubber material is raised to high temperature before the rubber material is poured into the cavity of a metal mold. In this method, however, the vulcanized rubber suffers from lowered modulus due to burning and/or the shear forces applied thereto during processing or working thereof, because before being poured the rubber material is subjected to high temperature. The lowered modulus of the vulcanized rubber will lead to lowering the durability of an engine mount as an end product.